Free radical halogenation is a well known reaction that is often carried out under a variety of conditions. The use of light, heat or microwaves have been used to generate reactive halogen radicals which quickly react with a variety of organic compounds to yield halogenated derivatives. However, it is the reactivity of the halogen radicals which cause problems with selectivity. Often, in addition to the desired monohalogenated derivative, higher polyhalogenated derivatives are formed. This problem is generally dealt with by running the reaction only to low conversion and then separating the unreacted starting material and recycling it. By conducting the reaction only to low conversion, the ratio of monohalogenated product to starting material remains high thereby reducing the formation of higher halogenated products. The disadvantage to this process is that it requires a considerable investment in equipment to effect the recycling, and low productivity per unit investment.
A typical low conversion halogenation process of the prior art employs recycling of the starting material to achieve high yields of monohalogenated product, while minimizing the yields of undesirable polyhalogenated products. Typically in this process, the halogen feed is directed into the liquid substrate contained in the reaction pot. The reaction pot is heated and simultaneously irradiated with light to promote the formation of halogen radicals which subsequently react with the organic substrate. The low conversion is accomplished by introducing only a fraction of the halogen necessary to complete the reaction. This avoids polyhalogenation since as the reaction proceeds, product is formed which remains in the reaction pot and is thus available for reaction with the halogen radicals to form polyhalogenated derivatives. The unreacted starting material is separated from the monohalogenated product (typically only 5-10% of the mixture) and recycled. Larger amounts of halogen can be introduced to yield higher percentages of monohalogenated product only at the expense of increasing amounts of polyhalogenated derivatives which cannot easily be recycled.
U.S. Pat. Nos. 2,510,149 and 2,715,134 both disclose the chlorination of organosilicon derivatives in the vapor or liquid phase. These procedures however do not allow the efficient, continuous separation of product and starting material necessary for high conversion to product while minimizing yields of polyhalogenated by-products. U.S. Pat. No. 4,101,397 discloses a low conversion process for the halogenation of organosilanes which employs continual recycling of the unreacted organosilane to provide high yields. The feed rate of the reactants is used to control the reaction temperature to avoid a potentially violent reaction or explosion. Thus a need exists for an improved efficient and selective process for monohalogenation. The present invention provides such a process for selective high yield halogenation of particular organic compounds without recycling of the starting material.